I used to work at a major European telecoms company... Unlike everyone else I am not talking programming mistakes, these are generally physical fumbles:
One day I was doing a change control, I was scheduled to change some settings on the modulator of a satellite system providing internet access to a portion of the Middle East. I called the satellite operator and told them I wanted to do the scheduled work, as they would have to confirm I was outputting new configuration within the constraints of the contract.
I entered the change and re-initiated the modulation, the operative said he was seeing nothing. Now, because this signal was going to the middle east I couldn't see it in Europe and without substantial plumbing I couldn't tap in to the antenna. My heart started going, I was checking amplifiers, up-converters, everything and I couldn't see anything wrong. After a few minutes the client called panicking because the action should have taken moments, not minutes. After more confirming with the satellite company that I wasn't transmitting I checked back through my steps and eventually saw I had missed one crucial thing: when you changed certain parameters it muted the output! A quick few button presses and the patient man on the phone said "there it is!" and I can relax again.
Lessons learnt:
1) I should have noticed a critical LED on the modulator was not lit!
2) This is the reason change controls say "working period 10 minutes" (time taken 9m50s).
3) A good boss will defend you if you recognise your mistakes and don't f*ck up too badly.
4) Don't go for a quick drink with your brother before a night shift.
In the broadcast industry they say you aren't a real engineer until you've taken a TV network off air. Lets say I am very experienced, but my employers have never had a problem with me. This is probably because I have also seen people meltdown under the pressure of delivering live services to millions or even billions of viewers, but if you can keep cool you can deal with it.